The count below identifies party affiliations at the beginning of the first session of this Congress, and includes members from vacancies and newly admitted states, when they were first seated. Changes resulting from subsequent replacements are shown below in the "Changes in membership" section.

Senators were elected by the state legislatures every two years, with one-third beginning new six-year terms with each Congress. Preceding the names in the list below are Senate class numbers, which indicate the cycle of their election. In this Congress, Class 1 meant their term began in the last Congress, requiring reelection in 1886; Class 2 meant their term began in this Congress, requiring reelection in 1888; and Class 3 meant their term ended in this Congress, requiring reelection in 1884.